As electronics technology shrinks, the performance of devices based on conventional semiconductors will become more challenging due to fundamental physical limits and more complex and expensive lithography processes. For example, statistical fluctuation in dopant concentration will add significant variability in the threshold voltage (i.e., gate voltage for drain current switched on) among devices on the chip as the channel width shrinks well beyond 100 nm; higher electric fields due to smaller distances can lead to avalanche (breakdown) of electrons causing progressive damage to the device; and the concomitant shrinkage of characteristics features (i.e., gate oxide and depletion layer thickness) could lead to current leakage due to quantum mechanical electron tunneling effect. Devices based on alternative physical phenomena to attain electronic switching without dopant and low (preferably single) electron transport are of great interest. It has been long known over two decades that nanoscale metal island isolated by dielectric barrier is an attractive solution to fabricate switching device for logic and memory where the charge transport is regulated at single-electron level. The single-electron tunneling (SET) junction occurs due to low capacitance of the island to store charge causing a Coulomb blockade against the next electron insertion into the nanoscale island. The result is a highly non-linear current (I)-voltage (V) characteristics where the current abruptly increases over a threshold bias, VCB that overcomes the required Coulomb blockade energy. However, the blockade energy, U=0.5 EVCB (approximately 1 meV for 100 nm island) is very low requiring operation below 10K to avoid thermal fluctuations.
Recently, by replacing the lithographically patterned metal island with (usually Au) nanoparticle having diameter of approximately 3-10 nm, the threshold energy can be raised approximately 100 meV making it possible to obtain Coulomb blockade at room temperature. SET devices operating at room and low temperatures, such as transistors and negative-differential-resistance using single nanoparticle have been demonstrated. However, for a viable single-electron digital device the charging energy must be approximately 100 kT to avoid thermally induced random tunneling. Thus, for a practical SET device operating at room temperature, VCB must be approximately 2.5V, a 25-fold increase from currently achieved nanoparticle based devices. Extending the above idea to particles <1 nm, VCB>5 V has been demonstrated in one recent study. However, for d<1 nm, the blockade characteristics are significantly smeared due to high sensitivity to size variations (approximately d3) caused by energy quantization effects; the operating currents drop by 103 fold in <1 pA as particle size decreases from about 1.8 nm to 0.7 nm, and charge fluctuations lead to significant drift in the I-V characteristic features over time. Based on theoretical calculations, a one dimensional necklace of larger nanoparticles could be an ideal structure to achieve higher switching voltages, with higher currents.
Electronic devices at nanoscale dimensions have the potential of achieving high performance with significantly lower power consumption compared to current devices. To reach this goal, existing studies on individual nanodevices must be translated from the isolated device-level to the circuit-level. Creating a circuit-level nanodevice system by simply scaling the silicon-based complementary metal oxide semiconductor (CMOS) devices to the nanoscale level is not possible due to entropy-driven fluctuations in the concentration of the dopant, which increases variability in device characteristics. Thus, dopant-free devices are of great interest. Logic circuits of nanodevices using one-dimensional nanostructures such as carbon nanotubes and doped semiconducting nanowires have been demonstrated. Although nanotubes do not require dopant, they are not very pure chemically—a batch of synthesized nanotubes is mixture of conducting, semiconducting and insulating tubes that are not possible to separate at the present time. Furthermore, the determination of their electrical properties is only obtained once the device is fabricated. Nanowires, on the other hand, have to be doped to build device.